ios学习笔记之block在ios开发中的应用
一、什么是Blocks
Block是一个C级别的语法以及运行时的一个特性，和标准C中的函数（函数指针）类似，但是其运行需要编译器和运行时支持，从ios4.0开始就很好的支持Block。
二、在ios开发中，什么情况下使用Block
Block除了能够定义参数列表、返回类型外，还能够获取被定义时的词法范围内的状态（比如局部变量），并且在一定条件下（比如使用__block变量）能够修改这些状态。此外，这些可修改的状态在相同词法范围内的多个block之间是共享的，即便出了该词法范围（比如栈展开，出了作用域），仍可以继续共享或者修改这些状态。通常来说，block都是一些简短代码片段的封装，适用作工作单元，通常用来做并发任务、遍历、以及回调。
三、block如何申明（对比于c语言中的函数申明） 
四、c函数指正和blocks调用
int (*CFunc) (int a) 函数调用
int result = CFunc(10);
int (^BFunc) (int a) 函数调用
int result = BFunc(10);
五、__block 关键字
一个Block的内部时可以引用自身作用域外的变量的，包括static变量，extern变量或自由变量（定义一个变量的时候，如果不加存储修饰符，默认情况下就是自由变量auto，auto变量保存在stack中的。除了auto之外还存在register，static等存储修饰符），对于自由变量，在Block中只读的。在引入block的同时，还引入了一种特殊的__block关键字变量存储修饰符。
六、block的几个小例子
#import <Cocoa/Cocoa.h>
int main(int argc, char *argv[])
{
@autoreleasepool {
NSLog(@"Hello world");
void (^myblocks) (void) = NULL;
myblocks = ^(void) {
NSLog(@"in blocks");
};
NSLog(@"before myblocks");
myblocks();
NSLog(@"after myblocks");
int (^myblocks2) (int a, int b) = ^(int a, int b) {
int c = a + b;
return c;
};
NSLog(@"before blocks2");
int ret = myblocks2(10, 20);
NSLog(@"after blocks2 ret %d", ret);
//此处如果不加__block会报错
__blockint sum = 0;
int (^myblocks3) (int a, int b) = ^(int a, int b) {
sum = a + b;
return3;
};
myblocks3(20, 30);
NSLog(@"sum is %d", sum);
}
returnNSApplicationMain(argc, (constchar **)argv);
}
打印结果如下
2012-09-03 10:23:20.878 blockTest[407:403] Hello world
2012-09-03 10:23:20.880 blockTest[407:403] before myblocks
2012-09-03 10:23:20.881 blockTest[407:403] in blocks
2012-09-03 10:23:20.881 blockTest[407:403] after myblocks
2012-09-03 10:23:20.882 blockTest[407:403] before blocks2
2012-09-03 10:23:20.882 blockTest[407:403] after blocks2 ret 30
2012-09-03 10:23:20.882 blockTest[407:403] sum is 50
七、block写的回调例子
1、Dog.h
#import <Foundation/Foundation.h>
@interface Dog : NSObject {
int _ID;
NSTimer *timer;
int barkCount;
//定义一个blocks变量
void (^BarkCallback) (Dog *thisDog, int count);
}
@property (assign) int ID;
//向外暴露一个接口
-(void) setBark:( void (^) (Dog *thisDog, int count) ) eachBark;
@end
2、Dog.m
#import "Dog.h"
@implementation Dog
@synthesize ID = _ID;
-(id) init
{
self = [superinit];
if (self) {
//每隔1s调用一次updateTimer方法
timer = [NSTimerscheduledTimerWithTimeInterval:1.0ftarget:selfselector:@selector(updateTimer:) userInfo:nilrepeats:YES];
}
returnself;
}
-(void) updateTimer:(id) arg
{
barkCount ++;
NSLog(@"dog %d bark count %d", _ID, barkCount);
//向Person对象进行汇报
if (BarkCallback) {
//调用从Person传过来的Blocks
BarkCallback(self, barkCount);
}
}
-(void) setBark:(void (^)(Dog *, int))eachBark
{
[BarkCallbackrelease];
BarkCallback = [eachBark copy];
}
-(void) dealloc
{
[BarkCallbackrelease];
[superdealloc];
}
@end
3、Person.h
#import <Foundation/Foundation.h>
#import "Dog.h"
@interface Person : NSObject
{
Dog *_dog;
}
@property (retain) Dog *dog;
@end
4、Person.m
#import "Person.h"
@implementation Person
@synthesize dog = _dog;
-(void) setDog:(Dog *)dog
{
if (_dog != dog) {
[_dogrelease];
_dog = [dog retain];
[_dogsetBark:^(Dog *thisDog, int count) {
NSLog(@"person dog %d count %d", [thisDog ID], count);
}];
}
}
-(Dog *) dog
{
return_dog;
}
-(void) dealloc
{
self.dog = nil;
[superdealloc];
}
@end
5、Main.m
#import <Foundation/Foundation.h>
#import "Person.h"
#import "Dog.h"
int main(int argc, constchar * argv[])
{
@autoreleasepool {
// insert code here...
NSLog(@"Hello, World!");
Person *person = [[Personalloc] init];
Dog *dog = [[Dogalloc] init];
[dog setID:10];
[person setDog:dog];
[dog release];
while (1) {
[[NSRunLoopcurrentRunLoop] run];
}
[person release];
}
return 0;
}
6、打印结果
2012-09-03 11:21:08.551 blockDelegate[549:403] Hello, World!
2012-09-03 11:21:09.554 blockDelegate[549:403] dog 10 bark count 1
2012-09-03 11:21:09.555 blockDelegate[549:403] person dog 10 count 1
2012-09-03 11:21:10.554 blockDelegate[549:403] dog 10 bark count 2
2012-09-03 11:21:10.555 blockDelegate[549:403] person dog 10 count 2
2012-09-03 11:21:11.553 blockDelegate[549:403] dog 10 bark count 3
2012-09-03 11:21:11.554 blockDelegate[549:403] person dog 10 count 3
2012-09-03 11:21:12.554 blockDelegate[549:403] dog 10 bark count 4
2012-09-03 11:21:12.555 blockDelegate[549:403] person dog 10 count 4
2012-09-03 11:21:13.553 blockDelegate[549:403] dog 10 bark count 5
2012-09-03 11:21:13.553 blockDelegate[549:403] person dog 10 count 5
2012-09-03 11:21:14.553 blockDelegate[549:403] dog 10 bark count 6
2012-09-03 11:21:14.554 blockDelegate[549:403] person dog 10 count 6